Monastir's medina is smaller and more manageable than those of Tunis or Sousse, and less overwhelmed by the tourist trade. The 9th-century city walls, partially restored, still define the boundary of the old city, and walking along or just inside them gives a clear sense of the original settlement's shape. The streets inside are quiet and genuinely residential — you are more likely to encounter local daily life here than in the heavily touristed medinas elsewhere on the coast.
The souks around the central mosque sell the usual mix of crafts, textiles, and spices. The fish market near the harbour is worth visiting early in the morning when the catch comes in. The medina is compact enough to explore thoroughly in a couple of hours; combine it with the ribat and the Bourguiba mausoleum for a satisfying half-day.